The 1,1'-disubstituted-4,4'-dipyridinium salt, also known as a "viologen" (Formula I) possesses one of the lowest cathodic reduction potentials of any organic system showing significant reduction-oxidation (redox) potential reversibility. The colorless salt form shown in Formula I can accept an electron and thereby be reduced to a deep blue-violet colored delocalized radical cation of Formula II. The radical cation of Formula II is readily oxidized back to the dicationic form of Formula I, for example, by atmospheric oxygen. Alternatively, the radical cation of Formula II can be further reduced to the red colored neutral form of Formula III, which can also be oxidized back to the starting dicationic form of Formula I. ##STR2##
Because of this remarkable redox behavior, viologens have been extensively used as photochemical or redox indicators in both chemical and biochemical applications. Additionally, viologens have been used for the estimation of dissolved oxygen in biochemical applications and as the main components of electrochromic display devices. See, for example, C. L. Bird and A. T. Kuhn, "Electrochemistry of the Viologens", Chem. Soc. Rev. 1981, 10, 49.
Particularly significant is the ability of viologens to function as electrochemical mediators, in that the viologen of Formula I can be electrochemically reduced to the radical cation of Formula II, and this radical cation can then reduce species which, under the conditions of the reaction, are not themselves susceptible to electrochemical reduction. For example, T. Endo, K. Ageishi, and M. Okawara in J. Org. Chem. 1986, 51, 4309 describe the zinc induced reduction of acrylonitrile in the presence of viologen derivatives. The electrochemical mediating ability of viologens has also been demonstrated to have applications in the catalytic photolysis of water where light is converted into chemical energy (I. Okura, H. Fujie, S. Aono, T. Kita, and N. Kaji, Bull. Chem. Soc. Jpn. 1986, 59, 309).
These pyridinium salts or viologens of Formula I have also been incorporated into polymers with the intent of taking advantage of the useful properties of the dipyridinium unit but in a material form that is easier to handle and separate from other components of the system, and to fabricate into desirable forms such as films, fibers, or membranes. For example, U.S. Pat. Nos. 3,641,034 (1972), 3,694,384 (1972), and 3,856,714 (1974) teach the incorporation of dipyridinium units into the main chain of polymers for use as redox polymers and electrochromic light filtering devices. Electrically conductive complexes of pyridinium polymers and 7,7',8,8'-tetracyanoquinodi-methane (TCNQ) are described by Sanada, et al, Nippon Kagaka Kaishi 1974, (5), 961 (Chem. Abstt. 1974, 81, 106154r), and by Sherle, et al., Vysokomol. Soedin., Ser. A 1974, 16(a), 2051 (Chem. Abstr. 1975, 82, 86976a). Pyridinium polymers useful as nonfoaming antistatic agents for the antistatic treatment of film are described in Ger. Offen. 2,544,841 (Chem. Abstr. 1977, 86, 24443y). Incorporation of pyridinium polymers into ion-exchange membranes is disclosed in U.S. Pat. No. 4,119,581 (1978), and use of pyridinium polymers as mordants in color diffusion transfer photography is taught in Japan. Kokai 77,155,528 (1976) (Chem. Abstr. 1978, 89, 207248v).
In general, these pyridinium polymers where the pyridinium unit is contained in the main chain or backbone of the polymer are prepared by a Menschutkin type reaction, that is, condensation of 4,4'-dipyridyl with an .alpha.,.omega.-dihaloalkane (See, A. Factor and G. E. Heinsohn, J. Polym. Sci. Polymer Lett. Ed. 1971, 9, 289.), for example: ##STR3##
Although this method does afford the desired useful pyridinium polymers, it has been reported (M. S. Simon and P. T. Moore, J. Polym. Sci., Polym. Chem. Ed. 1975, 13, 1) that films cast of the polymers obtained from this type of Menschutkin reaction were brittle and difficult to work with.
As an alternative to the Menschutkin reaction (that is, alkylation of an existing pyridine ring) another method for the preparation of pyridinium compounds is to utilize the reaction of a pyrylium salt (IV) with an amine (see A. R. Katritzky, R. H. Manzo, J. M. Lloyd, and R. C. Patel, Angew. Chem. Int. Ed. (English) 1980, 19, 306): ##STR4##
An example of the use of a bis(pyrylium) salt of structure V to prepare a polypyridinium of structure VI has been reported: Dzaraeva, et al., Khim. Geterotsikl. Soedin. 1985, (9), 1268 (Chem. Abstr. 1986, 104, 169303c). ##STR5##